1. Technical Field
The present invention generally relates to communications systems and, more specifically, to an architecture for communications systems which include fiber optic and coaxial distribution lines.
2. Description of Related Art
In the past, broadband coaxial cable television systems have been designed with a system architecture known as "trunk and feeder." The function of a trunk coaxial cable is to deliver broadband television signals from a reception center, or headend, to a plurality of distribution points. The distribution points are connected to feeder coaxial cables which emanate from the trunk coaxial cable and contain subscriber tap off devices. At the distribution points, the feeders connect to the trunk at locations commonly termed trunk/bridger stations. These bridger locations (along with the headend and any hubs) are generally known as "star" focal points with the feeder cables emanating in all directions therefrom.
In recent years, there has been a great deal of interest in the transmission of various types of information including, for example, television signals, via optical fibers. Optical fibers intrinsically have more information carrying capacity than do the coaxial cables which are used in present cable television systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals. Consequently, optical fibers are capable of spanning longer distances between signal regenerators or amplifiers than are coaxial cables. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by electrical shorting or radio frequency pick-up. Finally, optical fiber is immune to ambient electromagnetic interference ("EMI") and generates no EMI of its own.
It would therefore be desirable to provide an optical fiber cable to the home of each subscriber in a cable television system. Such a fiber-to-the-home (FTTH) architecture provides the above-identified advantages of using fiber as well as providing sufficient bandwidth to each customer for anticipated data services. However, the costs of such an arrangement are currently prohibitive.
Optical fibers have been implemented in various architectures. One such architecture is generally shown in FIG. 1 and is known as a fiber-to-feeder (FTF) system. In its simplest form, an FTF system replaces the coaxial trunk system with optical fiber to what was a bridger location. Thus, in the FTF system of FIG. 1, a headend 10 is coupled to a plurality of fiber optic nodes 15 via optical fibers 20. Fiber optic nodes 15 each include a transducer for transducing the received optical signals to corresponding RF signals which are output to a conventional coaxial plant which includes coaxial distribution lines 25 and a plurality of RF amplifiers 30.
To provide higher levels of service and performance to subscribers on a pocket by pocket approach, a fiber-to-serving-area (FSA) system such as generally shown in FIG. 2 may be utilized. In the FSA system, the coaxial trunk is replaced with a fiber optic supertrunk. At the fiber optic receive node, a high-output distribution amplifier is installed for feeding three or more express feeder cables extending further into the serving area. Signal distribution continues from there using distribution amplifiers in a normal feeder design. Thus, in the system of FIG. 2, a headend 31 is coupled to a fiber optic receive node 35 via a fiber optic cable 40. The output of fiber optic node 35 is supplied to a distribution amplifier 45. The output of distribution amplifier 45 is supplied to express feeder runs 50. These express feeder runs extend the reach of the amplifier cascades. Thus, as shown in FIG. 2, the serving area is further subdivided into mini-serving areas, each of which is fed by an express feeder.
A portion of an FSA system implemented by Scientific-Atlanta, Inc. of Norcross, Georgia is illustrated in FIG. 3. The system of FIG. 3 is a two-way communications system having a forward path with an RF frequency range of 54-750 megahertz (MHz) and a return path with an RF frequency range of 5-40 MHz. The forward path RF signals are transduced to optical signals at a headend (not shown) and transmitted over a fiber optic link 51 to a fiber optic node 52. Fiber optic node 52 passes approximately 500 homes. The optical signals received at fiber optic node 52 are transduced back to RF signals in a frequency range of 54-750 megahertz for distribution over a coaxial distribution plant. The output of fiber optic node 52 is supplied to a launch amplifier 53 which outputs signals to express feeders 56, 58, and 60. Express feeders 56, 58, and 60 and tap amplifiers 62 form part of the coaxial distribution plant. Other amplifiers such as express (or transport) amplifiers 59 for amplifying signals on the express feeders also form part of the coaxial distribution plant. Amplifiers 59 and 62 amplify both forward and return path signals over the coaxial distribution plant. Reference may be made, for example, to commonly assigned U.S. application Ser. No. 08/304,171, which is incorporated herein by reference, for a general description of diplex filter amplifiers which amplify both forward and return path signals. The return path signals from subscriber homes are supplied to fiber optic node 52 via the coaxial distribution plant. Fiber optic node 52 includes a reverse path transmitter for transducing the reverse path RF signals to optical signals for transmission to the headend via fiber optic link 51.
FIG. 4 is a detailed illustration of the portion of the system of FIG. 3 contained in box 61. Express feeder 60 from fiber optic node 52 and distribution amplifier 53 is coupled to tap amplifier 62 via an 8 dB directional coupler 64. Directional coupler 64 has a first output at which the signal level is reduced by 8 dB and a second output at which the signal level is reduced by 1 dB. The first output of directional coupler 64 is connected to tap amplifier 62 and the second output of directional coupler 64 is connected to a coaxial distribution line 66. Tap amplifier 62 outputs signals to coaxial distribution lines 68 and 70 and to two-way splitter 72. Splitter 72 divides the input signal into two equal parts which are supplied to coaxial distribution lines 74 and 76. Coaxial distribution line 76 feeds a 12 dB directional coupler 78 which couples a first portion of the signal supplied thereto to a coaxial distribution line 80 and a second portion of the signal supplied thereto to a distribution line 82. Various taps 84, 86, and 88 tap off a percentage of the broadband RF signal power on a corresponding coaxial distribution line to distribute to the subscribers' homes (not shown). Taps 84 may, for example, be two-way taps such as a Model SAT2F available from Scientific-Atlanta, Inc. Taps 86 may, for example, be four-way taps such as a Model SAT4F available from Scientific-Atlanta, Inc. Taps 88 may, for example, be eight way taps such as a Model SAT8F available from Scientific-Atlanta, Inc. Power is supplied to the distribution equipment via power lines 90 connected to telephone poles 92, power line poles 94, and transformers 96. Equalizers 98 may also be provided to equalize signal levels at various points in the distribution system.
Attempts are currently being made to migrate these and other hybrid fiber-coax (HFC) system architectures to architectures which push optical fiber deeper and deeper into the system, i.e., closer to the homes in the system. One attempt at migration from current HFC systems is a 1 gigahertz (GHz) fiber transmission system which was developed for use in a full service network in Orlando, Florida. Although this system provides for a return path from 900 MHz to 1 GHz, a guard band is needed from 735 MHz to 900 MHz to allow for reverse path signal amplification. This guard band actually reduces the available downstream bandwidth. Additionally, in this system, the performance of embedded drop cables must be sufficient to minimize a return path obstacle to interactive set-top terminals operating at low signal levels in the 900 MHz to 1 GHz band.
A so-called fiber-to-the curb (FTRC) architecture is shown in FIG. 5. It can be seen that while such an architecture pushes fiber optics deeper into the system, it requires a large number of optical fibers 91, fiber splice enclosures 93, and optical network units (ONUs) at the end of each optical fiber. Thus, this design results in significantly higher infrastructure costs.
Accordingly, there remains a need for a phased evolution of existing HFC systems which pushes optical fibers deeper into the communications system and which provides additional two-way bandwidth at relatively low cost.